docs: def cmd(self, value):
          """
          setter for final 'CMD' instruction in final build stage
  
          """
          cmd = None
          for insndesc in self.structure:
              if insndesc['instruction'] == 'FROM':  # new stage, reset
                  cmd = None
              elif insndesc['instruction'] == 'CMD':
                  cmd = insndesc
  
          new_cmd = 'CMD ' + value
          if cmd:
              self.add_lines_at(cmd, new_cmd, replace=True)
          else:
              self.add_lines(new_cmd)